1. Field of the Invention
This invention relates to a speaker which is thin but which can provide high-fidelity reproduction.
2. Description of the Related Art
There are great demands for speakers which are thin but can provide high-fidelity reproduction because of their space-saving characteristics. It is not very difficult to make a speaker thinner while maintaining its performance in the middle- and high-ranges of frequencies, above several hundred Hz. However, in the reproduction of low frequencies, below several hundred Hz, it is not possible to ensure an adequate sound pressure level unless the volume velocity of the diaphragm is increased. However, in a speaker system using a closed type enclosure, the stiffness of the enclosure S.sub.B (given by S.sub.B =.rho..sub.O C.sub.O.sup.2 S.sup.2 /V, where .rho..sub.O represents the density of air; C.sub.O represents the speed of sound; S represents the area of the diaphragm; and V represents the volume of the enclosure) is increased, so that the lowest resonance frequency f.sub.OB (given by ##EQU1## where S.sub.D represents the stiffness of the vibration system; and M represents the mass of the vibration system, including additional masses) of the system when the diaphragm is attached to the enclosure is increased; and the sound pressure level in the low-frequency range is thus reduced. To reduce the value of f.sub.OB, M may be increased or (S.sub.B +S.sub.D) may be reduced. However, if M is increased, the sound pressure level is reduced. Therefore, there is no alternative but to reduce (S.sub.B+S.sub.D). If S is made constant, f.sub.OB cannot be reduced because S.sub.B &gt;&gt;S.sub.D, since V is small in the thin speaker system. If S is reduced, it is necessary to increase the amplitude of the diaphragm in order to maintain a certain volume velocity. This may cause increased distortion.
For these reasons, conventional thin speaker systems are usually provided with rear-opening type enclosures. A system of this type will now be described with reference to FIG. 1. As shown in FIG. 1, a diaphragm 1 is connected to an enclosure 3f by an edge suspension member 2. The drive system, etc., are omitted in order to simplify the description. Such a speaker system was mounted on a rigid wall 4, and the axial sound pressure frequency characteristics obtained by varying the distance d between the system and the rigid wall 4 were measured. The results of these measurements are shown in FIG. 2. As is clear from FIG. 2, the sound pressure level in the low-frequency range increases as the distance d increases. This is because a sound A which is radiated from the front surface of the diaphragm and a sound B which is radiated from the rear surface achieve opposite phases at a measuring point P, and so cancel each other, as the speaker system is brought closer to the rigid wall. Therefore, this speaker system cannot ensure a desired reproduction sound pressure level unless it is spaced away from the wall by 50 to 60 cm. This speaker system is thin but it cannot realize any space-saving effect.
To overcome this problem of the rear-opening enclosure, a type of system has been proposed in which an acoustic duct is formed so as to improve the phase difference (at best, equalize the phases) between the sounds radiated from the front and rear surfaces at the measuring point, even when the system is positioned in close contact with a rigid wall. FIG. 3 shows this type of system. An enclosure 3g has an opening 6 which ensures that a sound radiated from the rear surface of the diaphragm passes through the duct 6 and then through an acoustic passage 7 which is formed between the enclosure 3g and a rigid wall 4, the sound thereafter being radiated toward the front. The sound pressure level is thereby improved because the phase difference between the sounds radiated from the front and rear surfaces of the diaphragm is increased by a phase corresponding to a distance l, which should be compared with that displayed in the above-described example.
If, in this method, the area of the diaphragm is increased in order to reduce the amplitude of the motion of the diaphragm, the mass M.sub.a in the gap between the rear side of the enclosure and the rigid wall ##EQU2## where S.sub.D represents the area of the diaphragm; S.sub.p represents the area of the opening; and M.sub.P represents the mass of air in the acoustic duct) is increased, thereby reducing the output sound pressure level.